Electro-optical modulation of light beams such as laser beams is well known in general and is described in many textbooks such as Optical Electronics by A. Yariv, Holt, Rinehart and Winston Company, 1985, pp. 274-305. Most electro-optical devices are either bulk modulators or waveguide modulators.
Bulk modulators can handle large optical powers because the laser beam diameter can be of the order of the transverse dimension of the electro-optical crystal, which may be of the order of 1 cm. Consequently, the intensity or power density, expressed in Watts/cm.sub.2, of the beam and the crystal can be held below the optical damage limit, even for high optical power systems. The power lost upon entering, propagating through and exiting from the bulk modulator can be quite small. However, the voltage required for good depth of modulation in such devices is usually hundreds to thousands of volts. Bulk electro-optical modulators are now available commercially from several crystal manufacturers.
A waveguide modulator requires much smaller voltages, on the order of 5-10 Volts for maximum depth of modulation. However, the power handling capability of these devices is limited to a few milliwatts because the laser beam is confined in a narrow portion of the modulator, typically having a width of several microns. Signal losses suffered in coupling into and out of a waveguide device are usually 6-8 dB, which is often unacceptable. Waveguide modulators are also available commercially.
One method of reducing the voltage required to provide good depth of modulation with a light modulator is to place the electro-optical crystal in a Fabry-Perot optical cavity that includes several high reflectivity surfaces. Near a resonance of this cavity, the voltage required for a given depth of modulation is reduced by the finesse of the cavity. According to one definition, finesse is the ratio of voltage required to change the resonant frequency of the cavity from one resonance to an adjacent resonance (a phase shift of 2.pi.), divided by the voltage required to change the resonant frequency between two half power points on the same resonance peak. An equivalent definition of finesse in terms of optical parameters is presented in A. Yariv, Optical Electronics, op.cit., pp. 92-94. The finesse itself may be a large, dimensionless number F&gt;&gt;1. For example, if the reflecting surfaces of the Fabry-Perot optical cavity have reflectivities of R=0.9, the finesse will be about 30 so that the driving voltages are reduced from several hundred volts to smaller voltages, such as 10-30 volts.
One version of a Fabry-Perot optical cavity that uses an electro-optical crystal with two reflecting surfaces is disclosed by F. Gires and P. Tournois, Comptes, Rendu, Acad. Sci. (Paris) Vol. 258 (1964) pp. 6112-6115.
Operation of an optical cavity near resonance in order to reduce the voltage requirement for an electro-optical modulator has not been widely used in the past, for several reasons. A resonantly enhanced electro-optical modulator must be used with a single frequency laser signal, and this has been hard to obtain. Further, the optical frequency of the laser source must be sufficiently stable to stay close to the desired resonance of the electro-optical modulator. A non-planar, internally reflecting ring laser, as described by Kane et al. in U.S. Pat. No. 4,578,793 and by Kane in U.S. Pat. No. 4,829,532, is a nearly ideal source for use with a resonantly enhanced electro-optical light modulator.
Use of an etalon external to a laser optical cavity for purposes of fine tuning the cavity output frequency is disclosed in U.S. Pat. No. 4,174,504, issued to Chanausky et al., in U.S. Pat. No. 4,550,410, issued to Hakini et al., and in U.S. Pat. No. 4,805,185, issued to Smith. Use of an etalon or equivalent means within an optical cavity to achieve a similar purpose is disclosed in U.S. Pat. No. 4,081,760, issued to Berg and in U.S. Pat. No. 4,284,963, issued to Allen et al. In U.S. Pat. No. 4,241,997, issued to Chraplyvy for a laser spectrometer with frequency calibration, two separate optical paths are formed, one containing a sample cell and the other containing a static etalon, with the light along each path being chopped at a different frequency to produce separate spectra for each path. None of these references discloses use of a dynamically controlled electro-optical material, functioning as an etalon in a two-path optical cavity, to control the phase and amplitude of a combination signal that issues from the cavity.
What is needed is a light modulator with low insertion loss, relatively low voltage requirements and high power handling capability. This package should also be reasonably compact.